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Full text of "The manufacture and properties of iron and steel"

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298                         METALLURGY OF IKON'AND STEEL.
already been stated that the conditions of their existence are •uncertain, so that for practical purposes these'two forms may be neg^ • lected until their properties have been further studied, and since the conditions under which austenite is formed are never realized in practice, this also may be passed by. Ferrite and cementite present very nearly the same appearance, but they never occur together, and as they differ very much in hardness it is easy to distinguish them, for ferrite is pure iron and if the point of a needle is drawn across it the surface will be easily scratched, while cementite is a compound of carbon and iron and the point will make very little impression. It is generally admitted that ferrite is structureless even under the highest powers of the microscope,
Pearlite is an "eutectic alloy," a term which may possibly not be familiar to all readers. An eutectic alloy is formed by the simultaneous crystallization of different metals in a liquid mixture, as for example a mixture of copper and silver. These metals form an alloy in the proportions of 72% silver and 28% copper at a temperature of 770° 0. (1418° F.), and if a melted mixture of these two metals contain any different proportion than this, and if it be allowed to cool, the element in excess of this proportion crystallizes but, the crystals remaining uniformly distributed throughout the molten mass. When the critical point of 770° 0. is reached, the alloy of 72 silver and 28 copper becomes solid, and entrains the innumerable crystals of the excess element which have separated from the mother liquid. A little consideration will show that under the microscope the element solidifying first and the eutectic alloy will occupy areas exactly proportional to the original constitution.
In steel at high temperatures the same conditions exist as in the mass of silver and copper just described, save that the elements are in what* is called "solid solution," martensite at the lowest critical point going through a transition into ferrite and eementite. The element in excess separates by itself, and when the proper relation has been established the ferrite and cementite crystallize together in most intimate mixture to form pearlite. As stated previously, the excess of cementite or ferrite begins to form by itself •at the upper critical point, a small'amount being found in steel quenched just below this, and at the second point this amount is increased, but this excess is always small except in the case of low carbon steel.                                                                               '